Solid electrolyte oxygen sensors include as a sensor element oxygen ion-conductive solid electrolyte (usually a sintered body), which is ordinarily provided on both its sides with porous electrodes. When the respective electrodes are exposed to a reference oxygen source (usually air) and an object to be measured, there is an electric potential difference in response to a difference in the partial oxygen pressure between both electrodes. The potential difference is guided to terminals where it is detected.
The sensors of this type are used especially for the measurement of oxygen (or an unburnt combustible matter) in exhaust gases discharged from internal combustion engines such as those for automobiles. The solid electrolyte used for that purpose includes those based on zirconia, hafnia or the like; however, they can work only in high-temperatures of about 400 degrees C. or higher, and their working temperature tends to rise further with the lapse of time during use. For this reason, when the temperature of exhaust gases is low, the sensor does not function well. For example, Japanese Patent Kokai Publication No. 55(1980)-69048 teaches an assembly wherein a ceramic insulating tube having a heating coil attached to one end is inserted into the inner bottom of a solid electrolyte element in the form of a bottomed cylinder, or a modification of such an assembly which further includes a heater arranged around that element.
In the conventional heater-incorporating oxygen sensors as mentioned above, however, a separately prepared, small-sized heater is merely attached to the end portion of the element, so that the overall structure of the sensor is complicated (provision of a heater-supporting structure or protective insulator such as an insulating tube, special leads and assembling means) with an increased number of assembling steps. This leads to increased production cost. It is further noted that, when the heater is mounted on the outside of the element, there are certain deteriorations of the heater.